1. Field of the Invention
The present invention generally relates to light-emitting devices, and more particularly to an electroluminescent device and a method of driving the same.
2. Description of the Related Art
FIG. 1 shows a first related-art organic electroluminescent device. This device includes a panel 100, a controlling circuit 102, a scan driving circuit 104, a discharge circuit 106, a precharge circuit 108, and a data driving circuit 110.
The panel 100 includes a plurality of sub-pixels (E11 to E44) formed in an area of crossed data lines (D1 to D4) and scan lines (S1 to S4). Each sub-pixel corresponds to a red sub-pixel, a green sub-pixel, or a blue sub-pixel, and each pixel comprises red, green, and blue (RGB) sub-pixels.
The controlling circuit 102 receives display data input from an external source. The display data may, for example, be RGB data. Circuit 102 controls operation of the elements in the organic electroluminescent device by using the received display data. The scan driving circuit 104 is formed in one direction of the panel 100, and transmits in sequence scan signals to the scan lines (S1 to S4).
The discharge circuit 106 includes a switch (SW) and a zener diode (ZD). The switch (SW) is turned on or off by a control signal from the controlling circuit 102. For example, when the data lines (D1 to D4) are discharged, the switch (SW) is turned on. As a result, the data lines (D1 to D4) are connected to the zener diode ZD, and a charge on the data lines (D1 to D4) is discharged up to a zener voltage of the zener diode (ZD).
The precharge circuit 108 applies a precharge current corresponding to the display data to the data lines (D1 to D4) in accordance with control of the controlling circuit 102. The data driving circuit 110 applies a data current corresponding to the display data to the data lines (D1 to D4) in accordance with control of the controlling circuit 102.
FIG. 2A and FIG. 2B show circuits for driving the organic electroluminescent device of FIG. 1, FIG. 2C is a timing diagram showing how the pixels of FIG. 2A and FIG. 2B are controlled to emit light. A first resistance (RS) between the outmost sub-pixel and ground has a value of 10Ω. A second resistor (RP) between sub-pixels has a value of 2Ω. Moreover, each of pixel (E41) and pixel (E42) emits light having a brightness corresponding to the data current of 3 amps. Further, sub-pixels (E11, E21 and E31) do not emit light. In addition, each of sub-pixels (E12, E22 and E32) emit light having a brightness corresponding to the data current of 1 amp.
To cause sub-pixels E11 to E41 along scan line S1 to emit light, precharge circuit 108 applies a precharge current corresponding to the display data to the E11 to E41 sub-pixels. (See FIG. 2A.) As a result, a charge corresponding to a second voltage (V2, default precharge voltage) is precharged to the E41 sub-pixel during a first precharge time (pcha1), as shown in FIG. 2C.
Subsequently, data currents (I11 to I41), which are 0, 0, 0, and 3 amps respectively, are applied to the data lines (D1 to D4). In this case, an anode voltage (VA41) of the E41 sub-pixel is increased up to a third voltage (V3), corresponding to the sum of a cathode voltage (VC41) and a voltage of 4V corresponding to a data current of 3 amps during T1 time. Then, the anode voltage (VA41) reaches a stable third voltage (V3) after a certain time. Here, the cathode voltage (VC41) is the whole current (sum of 0, 0, 0 and 3 amps) passing through the first scan line (S1) times a resistor of the scan line (sum of 10, 2, 2 and 2Ω), i.e. 48V, and thus V3 is 52V. Accordingly, the E41 sub-pixel emits a light having gray scale corresponding to 4V, i.e., the difference between the anode voltage (VA41) and the cathode voltage (VC41).
As shown in FIG. 2B, the precharge circuit 108 applies a precharge current corresponding to the display data to the E12 to E42 sub-pixels. As a result, a charge corresponding to the second voltage (V2, default precharge voltage) is precharged to the E42 sub-pixel during a second precharge time (pcha2), as further shown in FIG. 2C.
Subsequently, data currents (I12 to I42), which respectively correspond to 1, 1, 1, and 3 amps, are applied to data lines (D1 to D4). In this case, an anode voltage (VA42) of the E42 pixel is increased up to a fourth voltage (V4) corresponding to the sum of a cathode voltage (VC42) and the voltage of 4V corresponding to the data current of 3 amps during T2 time. Then, the anode voltage (VA42) reaches a stable fourth voltage (V4) after a certain time. Here, the cathode voltage (VC42) is the whole current (sum of 1, 1, 1 and 3 amps passing through the second scan line (S2) times the resistor of the scan line (sum of 10, 2, 2 and 2Ω), i.e. 96V, and thus V4 is 100V.
In summary, the difference of the stabilized anode voltage (VA42) of the E42 sub-pixel and the precharge voltage (V2) is higher than that of the stabilized anode voltage (VA41) and the precharge voltage (V2). Hence, T2 is bigger than T1. As a result, the consumed amount of charge to stabilize anode voltage (VA42) in the E42 sub-pixel is higher than is required to stabilize anode voltage (VA41) in the E41 sub-pixel, as shown in FIG. 2C. Accordingly, the E42 sub-pixel is designed to emit light at the same gray scale level as the E41 sub-pixel, but in reality emits light having a gray scale level smaller than the E41 sub-pixel. This phenomenon is often referred to as a cross-talk phenomenon.
FIG. 3 shows a second related-art organic electroluminescent device. This device includes a panel 300, a controlling circuit 302, a first scan driving circuit 304, a second scan driving circuit 306, a discharge circuit (e.g., a circuit to ground), a precharge circuit 310, and a data driving circuit 312. (Since the elements of this embodiment except the first scan driving circuit 304 and the second scan driving circuit 306 are the same as those of the first embodiment, any further detailed descriptions concerning the same elements will be omitted.)
The first scan driving circuit 304 transmits first scan signals to one group of scan lines (S1 and S3) in one direction of the panel. The second driving circuit 306 transmits second scan signals to remaining ones of the scan lines (S2 and S4) in other direction of the panel. As in the first related-art organic electroluminescent device, the cross-talk phenomenon occurs in the second related-art organic electroluminescent device. Also, the light-emitting process in the second device is similar to the device, and thus any further detailed descriptions concerning the process will be omitted.